feat: rate limiting + internal health monitor + secure VAPID note

Rate limiting:
- Shop POST endpoints get per-IP throttles with distinct prefixes so login,
  register, password-email, and password-reset have separate buckets:
  login/register/pw-reset = 5/min, pw-email = 3/min
- OcrInvoiceService gates per-tenant via RateLimiter (30/hour) so a runaway
  uploader can't burn Claude Vision spend

Health monitor (poor-man's monitoring):
- HealthCheckCommand probes DB (SELECT 1), cache write/read, public storage
  write/read, and most-recent backup age. On any failure, pushes a Telegram
  alert via HEALTH_ALERT_BOT_TOKEN/HEALTH_ALERT_CHAT_ID. Dedups identical
  failures within a 30-min window via cache.
- Scheduled every 10 min. Pair with external uptime monitoring (UptimeRobot,
  Better Stack hitting /up) for total-outage coverage.
- .env.example documents the two new env vars.

VAPID secret hygiene:
- credentials.md no longer stores the VAPID_PRIVATE_KEY; the source of truth
  is the Coolify env on the autocrm app. Doc points to where to read it
  (UI or API). Mitigates accidental git leak.

Tests (4 new):
- shop login throttles after 5 attempts (6th = 429); register throttle is
  independent of login (separate prefix); health command runs clean; dedup
  cache path exercised

Full suite: 138 passed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.env.example

@@ -63,6 +63,12 @@ VAPID_SUBJECT=mailto:admin@service.mir.md
 VAPID_PUBLIC_KEY=
 VAPID_PRIVATE_KEY=
 
+# Internal health monitor → Telegram alerts every 10 min on DB/cache/storage/backup failure.
+# Create a separate bot at @BotFather and a private group; put the bot in it
+# and use the group's chat_id (negative number).
+HEALTH_ALERT_BOT_TOKEN=
+HEALTH_ALERT_CHAT_ID=
+
 # Storage — local pentru MVP, S3-compatible mai târziu
 FILESYSTEM_DISK=local